This invention relates generally to operational amplifiers, and more particularly to a quad-operational amplifier layout characterized by thermal as well as stress symmetry.
As is well known, the power dissipated by a power device on an integrated circuit, (for example, an output transistor of an operational amplifier) results in the creation of isothermal contours about the power device. Clearly then, any devices in the circuit which must be critically matched (e.g. the input devices) should be positioned on the same isothermal contour to reduce thermal mismatch resulting in unwanted offset voltages. Therefore, in a single operational amplifier, devices which require critical matching are positioned equidistant from the power devices and from the layout's axis of symmetry.
This type of positioning has also been employed in the layout of a dual operational amplifier. That is, each cell or channel is the mirror image of its adjacent cell and shares the same axis of symmetry. In this manner, not only do the critical devices of each cell reside on the same isothermal contour created by its associated power devices, but they also reside on the same isothermal contour created by power devices in the other cell. If the power devices of both cells are simultaneously generating power, the resultant isothermal lines are created by the superposition. Even in this case, the critical devices reside on the same isothermal contour. Thus, the dual operational amplifier may be characterized as having channel-to-channel symmetry.
Unfortunately, standard configurations for quad-operational amplifiers are such that each cell or channel occupies one quadrant of a four quadrant layout. While symmetry is maintained in each cell, channel-to-channel symmetry is lost producing unwanted offset voltages and reducing desired channel separation.
Furthermore, currently manufactured quad-operational amplifiers are not suitable for packaging in small outline integrated circuit packages without substantial modifications, some of which result in significant disadvantages.